Recently, some kinds of high performance structural steel have been developed by the iron manufactures in Japan. In this research project concerning with the development on structural functions of steel structures the high strength steels with low yield ratio (LYR steel) were focused, and the material properties and structural behaviors of LYR steels were surveyed by the experimental and numerical investigations.The following results can be obtained :1. Material Properties of LYR SteelA data-base on the material properties such as yield ratio, uniform elongation, hardening strain, etc. of LYR steels was framed by the inquiries for steel manufactures and lots of tensile test. For LYR 60 steel which has 60kgf/cm^2 static tensile strength, the material properties were evaluated statistically.2. Hysteretic Performance of LYR Steel under Uniaxial Compression-TensionDafalias-Popov's two surface plasticity model was examined by a series of cyclic loading test for several structural steels including LYR steels. Introducing the movement of bounding lines, the reduction of elastic range and the appropriate modification of shape parameter, the uniaxial cyclic behavior of test results can be successfully simulated.3. Cross-Sectional Strength and Deformation Capacity of I-Section Beans Using LYR SteelUsing the stress-strain relation representating LYR 60 steel chosed by the data-base, the comparative investigations by elasto-plastic finite displacement analysis were conducted. In case of compact sections the superiority in ductility of LYR steel to the customary high strength steel can be observed.4. Strength and Ductility of Frame Corners Using LYR SteelBox to box connection models of steel frames made from LYR steel and customary high strength steel were tested under cyclic loading. The difference on hysteretic behavior due to material could not be observed, and each models have almost same deformation capacity.